When a heart goes into fibrillation its muscles contract in a randomly related manner so that it cannot pump blood. In this condition the electrical charges that control the muscle action are improperly distributed, but if they are properly oriented, for even an instant of time, it is possible that the heart can resume normal operation. Reorientation may be accomplished by passing electrical current through the heart with an instrument known as a defibrillator. It operates by discharging current from a storage capacitor through paddle electrodes that are pressed against the body of a patient at certain locations relative to the heart.
During fibrillation, the timing of the pulse of defibrillation discharge current is unimportant, but once the heart acquires some degree of proper rhythm, an ill timed discharge can cause the heart to revert to a state of fibrillation and the patient may die. For this reason cardiographic signals are displayed on an oscilloscope. If the waveforms of heart action on the oscilloscope indicate a condition of fibrillation, the discharge can be initiated manually, but if they indicate a rhythmic condition, the discharge must be controlled automatically by synchronizing circuits so that it occurs at the correct point in the heart cycle. The signals for the oscilloscope, as well as for the synchronizing circuits, can be derived from the paddle electrodes, but signals of better quality can be derived from a set of ECG electrodes. The selection has been made by a manual switch and applied to an amplifier that is common to both the oscilloscope and the synchronizing circuits. The switch is expensive and bulky because it must withstand the discharge applied to the paddles, and the quality of the signals provided by the common amplifier is impaired by virtue of the fact that it must be designed so as to work with the paddles as well as with the ECG electrodes. Of greater importance, however, is the fact that if the switch is set to select signals from the ECG electrodes and one of them falls off or becomes maladjusted, as can easily occur under the circumstances in which a defibrillator is used, the signal falls at a time when it is vitally needed. If this is noticed, precious seconds are lost, but if it is not noticed, the consequent disappearance of the ECG waveform from the oscilloscope may be erroneously interpreted to mean that the patient has cardiac standstill, and efforts that might save his life may be misdirected, leading to death.